lib: improve system reboot and shutdown implementation

We improve sbi_system_reboot() an sbi_system_shutdown() by:

1. Calling halt IPI to all harts (except current HART) before
   calling platform reboot/shutdown hook.
2. Calling sbi_exit() instead of sbi_hang() in-case platform
   reboot/shutdown hook failed.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>